When vaccinating children against varicella (chicken pox), researchers at Yale School of Medicine have found, two doses are better than one. The study will appear in the February 1 issue of Journal of Infectious Diseases.
The U.S. Centers for Disease Control and Prevention (CDC) began recommending a single dose of chicken pox vaccine for children ages one to 13 in 1995. The chicken pox rate fell drastically and studies showed that the effectiveness of one dose was 86 percent. But there was still a high rate of breakthrough illness in immunized children. The CDC changed the immunization policy for chicken pox in 2006, adding a second dose for children ages four to six.
In the new study, leader author Eugene Shapiro, professor in the Department of Pediatrics at Yale and his colleagues at Yale and Columbia universities showed that the effectiveness of two doses is 98.3 percent.
Past studies have suggested that two doses of varicella vaccine are linked to higher antibody levels than one dose, but this is the first study to assess the clinical effectiveness of two doses of the vaccine in the general population. In a survey of Connecticut children, Shapiro and his team discovered 71 cases of chicken pox in children ages four and older. None of these children had received two doses of vaccine; 66 (93 percent) had received one dose and five (seven percent) had received no vaccine.
"We weren't surprised to find that two doses of varicella vaccine are highly effective and are more likely to prevent varicella than a single dose," said Shapiro. "The findings confirm that, at least in the short term, the policy of routinely administering two rather than one dose of varicella vaccine is sensible. Other countries that are routinely immunizing children with varicella vaccine may consider changing to a two-dose regimen. "
Because it has only been four years since the CDC policy change, Shapiro also recommends that there be continued monitoring of the effectiveness of two doses to assure that its high degree of effectiveness is sustained.